How long a Pure Power 430 lasts?

Hi guys. For a good quality PSU, is there a good "rule of thumb" to go by as to the usable life span?

I ask only because I am on the home stretch of building my 1155 system, and I currently have a Thermaltake psu Pure Power 430W that's getting to be about 4 years old. It's been powerering the system in my sig since purchased new. On average, I'd say my computer is on about 60 hours a week, with 50% of those hours used for gaming. The rest of the time it is either powered off or in Sleep Mode.

Should I consider a new PSU, or should the Pure Power 430W keep chugging along for quite some time yet?